There are two types of move you may be talking about. Regular moves like Scratch, Ember and so on have 100% accuracy but are affected by many things:
- accuracy-reducing moves like Sand-attack
- evasive moves like Double Team
- some two-turn moves like Dig or Fly (i.e. when the Pokemon has left the battlefield attacking moves will miss)
- hold-items such as BrightPowder (lowers the foe's accuracy)
- fog weather condition
There are also moves that cannot miss, like Swift. We call those "infinite accuracy" moves since the accuracy cannot be reduced by the things listed above.